1. Field of the Invention
The present invention relates to an energy storage wheel, or inertia flywheel, of the hybrid type, namely comprising portions made more particularly from a composite material and portions made from a non composite material.
2. Description of the Prior Art
The completely metal conventional inertia flywheel, formed by a ring (or hoop or rim) connected to a hub by means of spokes, (cf. FIG. 1 accompanying the present description), is outside the field of the present invention because it has a geometry which is not adapted to the use of composite fiber materials, composite material flywheels being made from circumferentially wound fibers impregnated with epoxy resin based materials.
The replacement of metal flywheels by those which use composite materials has brought the following advantages:
the possibility of storing a larger amount of energy per unit of mass of the flywheel, because of the higher resistance to tensile forces of the fibers, and
an improvement in the bursting behavior when the value which the tensile strength has at the bursting point of the fibers is accidentally exceeded ; in fact, if the fibers are acted on by forces beyond their bursting strength, instead of being reduced to large fragments, as in case of metal, they are reduced to small fragments , and eventually into powder, which is particularly important in the case of space application.
Composite material flywheels of the prior art include an original rimless construction in the form of a star, (cf. SCIENTIFIC AMERICAN vol. 229, No. 6, December 1973, pages 17 to 23 as well as FIG. 2 accompanying the present description), which comprises solely a plurality of spokes, each spoke being made from a composite material having the direction of the fibers parallel to the length of each spoke, so that the centrifugal forces induce only lengthwise radial stresses in the fibers, which are thus optimally stressed.
Although this solution avoids the problem of delamination, namely the transverse displacement of the fibers due to the forces acting perpendicularly to the longitudinal direction of the fibers and to the shear forces between fibers, it has the following disadvantages :
the volume containing useful material is relatively small because the spokes occupy only a fraction of the volume swept out by their free ends, so that for storing a given amount of energy the useful volume must be increased, which also increases the cost and limits the field of application
the spokes are unevenly stressed lengthwise, the portions highly stressed being those which are the closest to the hub, in which high moments are generated when the flywheel is spun up, which prevents making the best use of the performance of said fibers,
need to fix spokes to the hub by piercing the midpoints of these spokes creates serious problems because the cross section is reduced precisely where the stresses are highest.
Another known design (cf. again the above mentioned document SCIENTIFIC AMERICAN as well as FIG. 3 accompanying the present description) is a flywheel comprising a plurality of concentric rings made from a composite material and separated from each other by a clearance.
The concentric rings have a density which increases towards the center so as to improve the stress distribution, and each separation clearance is filled with elastomeric bonded bands allowing relative expansion of the different rings.
However, this second solution requires the use of filaments of thick fibers, which presents difficulties in terms of processes for manufacturing such fibers, particularly during the curing process. Furthermore, the structure is likely to induce delamination effects in the body of the rings.
More recent composite flywheel designs are aimed at optimizing the stresses induced by centrifugal forces in all the fibers . The geometrical configuration of such flywheels is such that most of the fibers are stressed radially up to a very high level, so as to make optimal use of these high mechanical strength materials under the best conditions, i.e. such that the stresses induced by the centrifugal forcesare directed along the length of the fibers .
A composite material flywheel satisfying this requirement comprises (cf. FLYWHEEL TECHNOLOGY SYMPOSIUM, October 1980, pages 4 to 12 as well as FIG. 4 accompanying the present description), an external ring which is composed of layers of carbon fibers embedded in an epoxy resin matrix and wound along the circumference of the ring, and an internal ring composed of windings of very high mechanical strength steel fibers which are also embedded in an epoxy resin matrix: the two rings are joined together by means of an appropriate bonding agent.
This double ring is connected to the hub by means of a carbon fiber/epoxy resin web which envelops the external ring and which plays the role of connecting "spokes".
Another flywheel corresponding to the above criterion also comprises (cf. again FLYWHEEL TECHNOLOGY SYMPOSIUM, October 1980, pages 168 to 173 as well as FIG. 5 accompanying the present description) a double ring composed of "Kevlar" fibers in its outer portion where the tangential speed is the highest, and "S" glass fibers, of higher density, in the inner portion.
This double ring is connected to the hub of the flywheel by means of a plurality of loops which are disposed between the internal part of the double ring and the hub and which play the role of connecting "spokes" in which the profile of their free portion is that of a polar catenary.
Such loops are made from a composite material having low density and also a low modulus of elasticity whose value is such that the tensile stress in the loops remains less than that in the ring.
In the last two designs which have been described, the differences in density and modulus of elasticity of the materials used for constructing said internal and external rings are such that, under the effect of the centrifugal force, the internal ring would expand more than the external ring if their expansion was able to take place freely, so that, because of the mutual contact existing between these rings, the external ring compresses the internal ring limiting the forces which are induced therein . This same criterion is applied to the second of these last two designs for constructing the hub which comprises an internal aluminum portion reinforced by an overwrap made from a composite "Kevlar"/epoxy resin material.
Now, in each of these last two designs, the "spokes" support a considerable portion of the load produced by the (double) ring when the flywheel is operating, so that the maximum value of the energy which can be stored is limited by the design of said "spokes".
Furthermore, the stability of the flywheels varies during acceleration and slowing down, which is a disadvantage more particularly in space applications.
The aim of the present invention is therefore to provide an inertia flywheel (or an energy storage wheel) which satisfies practical requirements more than previously known flywheels, in particular in that:
the maximum energy which may be stored per unit of mass of the flywheel of the invention , i.e. the specific energy of the flywheel, is substantially independent of the design of the "spoke" of the wheel, i.e. that it is not limited by the intrinsic resistance of the material from which these "spokes" are made,
the problem of delamination is substantially reduced and, in any case, the effects due to a possible delamination are eliminated, and
a larger amount of energy may be stored in a wheel of given diameter.